Improvements of charged particle beam devices, like electron microscopes, electron or ion beam inspection or pattern generating tools, e.g. focused ion beam devices (FIB), depend on further improvements of their beam optical components. Beam optical components include, for example, electrostatic or magnetic charged particle lenses, deflectors, beam apertures, charged particle beam sources and the like.
Charged particle lenses require a high degree of mechanical precision in order to obtain a focus spot of the smallest possible size, which is a prerequisite for obtaining the highest possible spatial resolution when inspecting or structuring a specimen. High precision focussed charged particle beams are used in charged particle beam devices like electron microscopes, pattern generators for lithographic processes in the semiconductor industry or focused ion beam devices (FIB).
Charged particle lenses usually use electrostatic or magnetic fields for focussing the charged particle beam. Charged particle lenses with electrostatic fields are usually composed of two or more electrodes that each have an opening through which the charged particle beam can pass. By applying appropriate voltages to the respective electrodes, the geometric shape of the electrodes and the electric potentials provide for an electrostatic field that can be used to focus an incoming charged particle beam.
Charged particle lenses with magnetic fields, in contrast, are composed of two pole pieces with openings through which the charged particle beam can pass. By providing an appropriate magnetic flux to the respective pole pieces, the geometric shape of the pole pieces and the magnetic flux determine a magnetic field that can be used to focus an incoming charged particle beam.
For a high focussing quality, it is important that the openings of the multiple electrodes or pole pieces are well aligned with respect to each other and with respect to the charged particle beam axis. For example, in order to obtain an electron or ion beam focus of a size smaller than 100 nm, the openings of the electrodes (or pole shoes) need to be aligned with respect to each other with a precision on a micrometer scale. Further, the smaller the openings, the smaller the alignment tolerances are.
A method for producing charged particle lenses with a high alignment precision is disclosed, e.g. by S. Planck and R. Spehr in “Construction and fabrication of electrostatic field lenses for the SMART project” in the Annual Report 1996/1997 of “Licht- und Teilchenoptik”, Institut für angewandte Physik, Technische Unversität Darmstadt, Prof. Dr. Theo Tschudi on page 114. S. Planck and R. Spehr use insulating precision spheres between electrodes to position the openings of the electrodes with respect to each other. However, for further progress in the focussing of charged particle beams, the alignment of the openings to each other and to the charged particle beam is often not sufficient.